A silicon nitride ceramic porous body having excellent acid and alkali resistance, mechanical strength, and durability can be employed as a filter or a catalytic carrier. The silicon nitride porous body contains a plurality of silicon nitride crystal grains with pores formed in grain boundary parts thereof, or includes a body part and a pore part wherein the body part is formed by a plurality of silicon nitride crystal grains and the pore part forms a three-dimensional network structure. The body part is formed by at least 90 vol. % of silicon nitride crystal grains, which are directly bonded to each other. In order to prepare the finished ceramic porous body, a porous body compact which is mainly composed of silicon nitride, is brought into contact with an acid and/or an alkali so that a component other than silicon nitride is partially or entirely dissolved and removed from the compact. The compact is prepared from a mixed powder of silicon nitride powder and at least one of a rare earth compound powder, a transition metal compound powder, and a bismuth compound, which is heat treated in the temperature range from 1600.degree. C. to 2100.degree. C.